(1) Field of the Invention
The present invention relates to an image processing method comprising the steps of converting input analog image signals into digital image signals, generating a density histogram of the pixels from the obtained digital signals and effecting density correction of the digital image based on the generated density histogram as well as to an image processing apparatus using the method. More detailedly, the present invention is to determine the background density and maximum density from the pixel density histogram of a target image and generate a density correction curve suited to the target image based on the determination.
(2) Description of the Prior Art
Widely used image forming apparatus such as electronic copiers are categorized into two classes, the conventional analog type and the digital type. Under such circumstances, typical analog copiers have been known to use an automatic exposure function, by varying the intensity of the exposure lamp based on the detection of the original density with an original density sensor, so as to provide optimal image quality. For digital copiers to realize such an automatic exposure function, there are proposals for automatic density control such as Japanese Patent Publication Hei 3 No.30143(prior art 1), Japanese Patent Application Laid-Open Hei 8 No.204963(prior art 2) and Japanese Patent Application Laid-Open Hei 9 No.43915(prior art 3), for example, in which optimization of the image quality is effected based on the density histogram of a target image.
In the automatic density control in the conventional image forming apparatus, especially in the invention of the above prior art 1, sampling is performed periodically and at local peaks to generate a density histogram, and the pattern of this histogram is compared with the data stored in the ROM (read only memory), to thereby determine the type of an original, e.g., white background printed original, color background printed original, white background pencil written original or the like so as to output an image density control signal corresponding to the determined type.
In the invention disclosed in the above prior art 2, the positions of two peaks, i.e., ‘white’ and ‘black’ peaks, are determined based on the density histogram generated from a target image so as to decide reference values while the type of the image is determined based on the density histogram so as to correct the reference values. Tone correction is performed based on the thus corrected reference values.
Further in the invention disclosed in the above prior art 3, when reference values for density correction are determined with reference to frequency values of the density histogram, the density histogram is generated only for the original area even if the document cover is open, whereby it is possible to perform automatic density control without it being affected by the data of the area outside the original.
Concerning the conventional automatic density adjustment methods in image forming apparatus, in the case of the invention written in the prior art 1, data in ROM is used as the means for determining the type of an original. Therefore, for detailed determination, it is necessary to store the comparable data, which results in the need of a very large capacity ROM. Further, there may be cases where determination itself is impossible for special images, which limits the applicability of the machine.
In the cases of the prior art 2 and 3, the reference values for tone correction are determined based on the density histogram of a target image, it is impossible to determine whether the target image is of photographs or of characters. Therefore, if the density histogram does not present a true density distribution of the original as in the case where the image is composed of photographs and characters or in the case where there are a plurality of background tones, it is impossible to calculate the reference values for optimal correction, thereby making the automatic density control impossible or producing defective images. Further, in the case of prior art 3, since the process to be done when the document cover is open differs from that when it is closed, the operation itself becomes complicated.
In addition, in conventional digital image forming apparatus, especially that incorporating a manual density control device, upon the density control operation the user should make a trial copy to obtain the optimal density, or the user adjusts the density relying on experience. In either case, unnecessary and wasteful copying would have been done.